grid ratio

ratio

[ra´she-o] (L.)

an expression of the quantity of one substance or entity in relation to that of another; the relationship between two quantities expressed as the quotient of one divided by the other.

A/D ratio adult versus developmental toxicity ratio; the ratio between the toxic effects of a substance on adults (humans or animals, especially pregnant females) and such effects on developing embryos or fetuses.

arm ratio a figure expressing the relation of the length of the longer arm of a mitotic chromosome to that of the shorter arm.

benefit-risk ratio a determination of the potential of a research study for positive effects on patients compared to the risks of doing harm.

cardiothoracic ratio on a chest radiograph, the ratio of the transverse diameter of the heart to the internal diameter of the chest at its widest point just above the dome of the diaphragm.

grid ratio a ratio comparing the height of lead lines in an x-ray grid to the distance between the lead strips.

inspiratory-expiratory ratio the ratio of the inspiratory time to the expiratory time during the breathing cycle. Normally, expiratory time will be longer than inspiratory time; if the inspiratory time is longer than the expiratory time, gas trapping may result.

sex ratio the proportion of one sex to the other; by tradition the number of males in a population to the number of females, usually stated as the number of males per 100 females.

signal-to-noise ratio the ratio between the amplitude of a signal being measured and that of the noise.

urea excretion ratio the ratio of the amount of urea in the urine excreted in one hour to the amount in 100 ml of blood. The normal ratio is 50.

zeta sedimentation ratio (ZSR) a measurement comparable to the erythrocyte sedimentation rate, except that it is unaffected by anemia. The packed-cell volume (zetacrit) of a blood specimens is calculated by centrifuging the specimen in a Zetafuge, a specially designed instrument that produces controlled cycles of compaction and dispersion and allows rouleaux to form and sediment rapidly. The zetacrit divided into the true hematocrit gives the zeta sedimentation ratio.

grid ra·ti·o

in a radiographic scatter-absorbing grid, the ratio of the height to the width of the gaps between lead strips; a higher grid ratio removes more scattered radiation but requires more careful x-ray tube positioning to avoid grid cutoff of the primary radiation beam.

grid ratio

(in radiology) the ratio of the height of the lead strips to the width of the interspacing of a grid.

grid

1. a grating; in radiology, a device consisting essentially of a series of narrow lead strips closely spaced on their edges and separated by spacers of low density material; used to reduce the amount of scattered radiation reaching the x-ray film.

2. a chart with horizontal and perpendicular lines for plotting curves.

grid cassette

a radiographic cassette with a grid permanently installed in it.

grid cutoff

excessive loss of radiation beam because of incorrect angulation between the tube and the lead strips.

grid diaphragm

a grid interposed between the film and the x-ray beam. See also (1) above.

grid factor

because of the filtering out of rays by the lead strips there is a loss of penetrating effect of the beam so that exposure time must be increased.

focused grid

the lead strips in the grid are sloped slightly inwards at the top so that the apertures more closely approximate the angle taken by the rays in a diverging beam.

linear grid

a cassette grid in which the radiopaque strips are parallel to each other.

grid map

a map marked by a grid of numbered intersecting parallel lines making it possible to identify particular locations numerically.

movable grid

one that moves during the exposure of the film to the x-ray beam: a Potter-Bucky grid.

nonfocused grid

see parallel grid (below).

parallel grid

the lead strips in the grid are parallel to each other and vertical to the plane of the film. This has the disadvantage that more diverging rays are absorbed by the grid at the edges of the film than at the center with a lowering of exposure there.

Potter-Bucky grid

a focused grid moved mechanically across the x-ray beam. Suited only to large installations. Called also Bucky and Potter-Bucky diaphragm.

pseudo-focused grid

something of the same effect as a focused grid is obtained by gradually lowering the height of the strips in the grid as they approach the edge of the grid.

grid ratio

the ratio between the height of the strips to the distance separating them. The greater the ratio the more rays will be filtered out.

reciprocating grid

one that moves during the exposure of the subject to the beam of radiation.

rotating grid

one that moves while x-rays are being generated.

stationary grid

one that is stationary while the film is being exposed so that there is a pattern of grid lines on the radiograph.

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